This invention relates to an improved process for the preparation of aryl polycarbonates. The present invention particularly relates to the use of bioxyanion type catalysts in transesterification polymerization of diarylcarbonates and dihydric phenol for the production of polyarylcarbonates with improved color and higher thermal stability.
These polyarylcarbonates are the commercial engineering plastics produced by the industry for replacing glass and metals in several areas. Polyarylcarbonate is processed by molding, extrusion and film-forming techniques for conversion of it to different types of useful products.
In the prior art similar compounds have been reported to act as catalysts in `group transfer` addition polymerization of acrylic monomers. In this context reference may be made to the following literature.
(1) O. W. Webster, W. R. Hertler, D. Y. Sogah, W. B. Farnham and T. V. Rajan Babu, J.Am.Chem.Soc., 1983, 105, 5706. PA0 (2) I. B. Dicker, G. M. Cohen, W. B. Farnham, W. R. Hertler, E. D. Lagann and D. Y. Sogah, Polymer Preprints 1987, 29, 106.
Industry produces polyarylcarbonates by interfacial and transesterification process. In the former, phosgene is condensed with bisphenol-A at an interface of methylene chloride/aqueous sodium hydroxide, under ambient conditions in the presence of catalysts either amine or its salts. The molecular weight of the polymer is controlled by the addition of monohydroxy aromatic compounds. Transesterification consists of melt condensation of diarylcarbonate and dihydric phenol at elevated temperatures in the presence of either basic or acidic catalysts. The molecular weight is regulated by the removal of a byproduct, phenol which is expedited by the application of heat and vacuum treatments. Basic catalysts such as alcoholates, phenolates and carboxylates of alkali metals are cited as useful catalysts for transesterifications. Reference may be made to British Patent 1,110,736 on the transesterification process.
There are inherent limitations in both the processes. The isolation of interfacial product, polyarylcarbonate from the reaction mixture, needs washings for the removal of byproduct and catalyst, recovery of solvent for recirculation and densification of product prior to fabrications. The transesterification reaction obviates these drawbacks associated with the interfacial process. But the high melt viscosity of the prepolymer causes problems in its further transesterifications towards the synthesis of high molecular weight polymer. By utilising specialised equipment designed to generate high surface for overcoming high melt viscosity limitations for condensations, the polymer of desired molecular weight is achieved by appropriate heat and vacuum treatments. Most of the alkaline metal based catalyst used in the prior art for transesterifications induce discoloration of the products, due to the high temperatures involved. Also the catalysts contain metals which are left as residues in the polymer.
The main object of the present invention is to provide an improved process for the preparation of polycarbonates using quaternary ammonium bioxyanions as metal free catalysts for the melt transesterification-polycondensation of diarylcarbonates and dihydric phenols.